1. Field of the Invention
The invention relates to a broadband optical distribution system for a local optical fibre network, comprising a main exchange which is connected to a plurality of subscriber's sites via optical fibres, the transmission system comprising at least one auxiliary oscillator for converting in the subscriber's site the optical transmission signal applied by the main exchange into a lower intermediate-frequency electrical signal. The contempory wavelength multiplex systems are examples of optical frequency-division multiplex systems in which the optical carriers are roughly separated relative to each other. Each of the carriers is modulated with, for example, a digital 560M bit/s signal. In view of the dimensions of the present transmission frames, which are located between 800-900 nm and 1300-1500 nm this implies that approximately 16 of such optical carriers are potentially available. The small spacings between two optical carriers are simply a waste of frequency range. A space of 25 nm between two optical carriers represent a frequency separation of 7500 GHz. All these empty spaces together represent a frequency range waste of 16.times.7500 GHz.
2. Description of the Prior Art
The above problem can be obviated by using a heterodyne optical transmission system such as the system described in, for example, IEEE Transactions M.T.T., vol. 30, no. 8, August 1982, pages 1138-1149. A heterodyne optical transmission system enables an optical frequency multiplexing, in which the spaces between the optical carriers can be kept small. A difficult factor in said heterodyne systems is the stability of the optical frequency of the laser diodes required therefor. In an optical heterodyne system the signal-carrying optical frequency F.sub.1 is combined at the input of a photodiode with a local oscillation frequency F.sub.o. The output current of the photo-diode then includes an intermediate-frequency carrier having a frequency of (F.sub.o -F.sub.1) MHz.
So as to obtain reasonable variants for these intermediate frequency carriers, for example 1 GHz, it is imperative that both F.sub.1 and F.sub.o are limited within, for example, 100 MHz. Given the fact that the frequency drift of a laser diode because of temperature fluctuations is approximately equal to 20 GHz/.degree.C., this is very difficult to realise. If, for example, F.sub.1 =300,000 GHz this means that the temperature must be stabilised to 0.01.degree. C. This implies that this temperature stabilisation must be realised in every subscriber's site, which means very high costs for each subscriber site.